The invention relates to a piston reciprocating thermal engine where compression ignition, limited pressure (Diesel) and steam (Rankine) cycles are realized using the same cylinders at the same time.
In limited pressure compression ignition (Diesel) engines liquid fuel is injected into the compressed air in the cylinders. The bulk of combustion occurs on the stoichiometric surface of a diffusion flame separating the high temperature oxygen rich and high temperature fuel rich regions. The presence of the high temperature oxygen rich zones leads to production of large amounts of nitrogen oxides (NOx) and the presence of the high temperature fuel rich zones to soot (particulate matter) production. Thus Diesel engines are regarded by some as being highly polluting. The additional feature of the engines is that the exhaust gases are hot and the air and cylinder walls require cooling which causes substantial heat losses decreasing the thermal efficiency of the engines. In Rankine cycle (steam) engines water is evaporated by heating and the generated steam used to produce mechanical energy. Diesel-Rankine cycle combination is often used to utilize the waste heat of the Diesel (top) engine but the Rankine cycle is typically realized in a separate (bottom) engine. An example of such solution is described for instance in the U.S. Pat. No. 5,133,298. Clearly this is a relatively expensive and potentially difficult solution.
In a number of patents for instance: U.S. Pat. Nos. 4,377,934; 4,406,127; 4,706,468; and U.S. Pat. No. 4,901,531 different versions of the bottom steam cycle are proposed in which at least one cylinder of the internal combustion engine is used as the steam power recovery device, which in fact is a modification of the classic combined cycle engines. In another patent: U.S. Pat. No. 4,433,548 the use of the same cylinders alternatively for combustion and steam power strokes is described. In yet another patent: U.S. Pat. No. 4,409,932 injection of the steam during the power stroke i.e. after combustion in a gasoline car engine is proposed. A negative feature of all these solutions is that the emissions of the combined systems are the same as for the internal combustion (top) engine. Many patents for example U.S. Pat. Nos. 3,761,019; 4,014,299; 4,027,630; 4,059,078; 4,391,229; 4,409,931; 4,844,028 describe steam injection into the inlet manifold of engines to control power, knock or NOx. However these solutions do not necessarily provide increased efficiency. Some patents: U.S. Pat. Nos. 3,948,235 and 4,913,098 propose to supercharge the engine using the steam either by a steam driven compressor or compressor and ejector. The efficiency increase of the solutions is relatively small so as it is for the solutions described in U.S. Pat. Nos. 4,301,655 and 4,402,182 where special cylinder heads where steam can be evaporated and injected into the engines are described since in these solutions only the cylinder head cooling heat is utilized in the power stroke. Premixing the fuel with water, vaporizing the mixture and using the evaporated homogeneous steam-fuel mixture as a modified fuel are described in the U.S. Pat. No. 4,909,192. However, to realize the full potential of the system the steam fuel mass ratio shall be in the range 1:3 and the steam temperature above 500xc2x0 C. The massive steam injection in the premixed mode can cause severe ignition problems whereas preheating of the fuel to the temperatures required even in the presence of water may lead to substantial fuel decomposition and coke formation in the fuel/steam system. Thus the solution does not seem to be feasible especially for heavy fuel operation.
The aforementioned goal is achieved using as much as possible of the waste heat i.e. the air and cylinder cooling heat including cylinder head cooling as well as the exhaust of the internal combustion (Diesel) engine to generate steam which is then injected back into the cylinders of the internal combustion (Diesel) engine close to the top dead center so, that the steam actively changes the combustion process without interfering with the ignition. In this way the maximum potential of the Diesel-Rankine cycle combination is achieved using the same cylinders and reciprocating mechanisms at the same time. This simplifies and reduces substantially the cost of the system. In addition the maximum temperatures in the cylinders obtained during the fully developed combustion are reduced which radically decreases NOx emissions and the mixing of combustion gases with remaining air enhanced during afterburning which reduces soot emissions. To achieve the goals of the invention over a range of engine loads the rate of water injection into the steam generating apparatus should be proportional to the average fuel flow, which maintains the proper engine power and steam mass injection balance. In another embodiment a small part of the steam, typically less than 10% of the fuel mass, is used for rapid atomization of fuel without using expensive pressure fuel injection systems.
Thus the invention provides a simple yet effective solution to long standing problems of the combined Diesel-Rankine cycle engines. The foregoing and other objects and advantages of this invention will appear from the following detailed description, taken in connection with the accompanying drawings of a preferred embodiment.